This invention relates to an improved method of forming thin, layered coatings on substrates by sol-gel processing and to coated articles formed thereby.
There is presently a growing need for protecting various substrates against corrosion and other environmental effects by the use of thin films resistant to these effects There is also a need to provide enhanced adhesion between such coated substrates and between coated and non-coated substrates with the use of commonly available adhesives such as epoxy, urethane, and acrylic adhesives. A need also exists for providing substrates with a greater resistance to surface abrasion and scratching.
The most common methods of applying protective films to substrates has been by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering and thermal oxidation. All have disadvantages, especially with regard to the low temperature preparation of dielectric films.
Another method for applying thin protective coatings to a variety of substrates is known as sol-gel processing. Sol-gel processing is a method whereby small molecules can be converted into polymeric or ceramic materials. Depending on the nature of the monomers, one can form anything between organic polymers such as polydimethylsiloxane and inorganic ceramics such as silicon dioxide. Typically one mixes metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, P, in an appropriate solvent such as an alcohol with water and a catalyst. In the catalyzed solution the alkoxides are partially or completely hydrolyzed and then polymerized to form molecules of a glass-like oxide or ceramic network linked by bridging oxygen atoms The overall process that takes place using tetraethoxysilane as the metal alkoxide involves two main reactions. The first is the hydrolysis of the alkoxide to form the hydroxy-containing species (eq. 1). EQU Si-(OCH.sub.2 CH.sub.3).sub.4 +H.sub.2 O.fwdarw.HO-Si(OCH.sub.2 CH.sub.3).sub.3 +CH.sub.3 CH.sub.2 OH
This reaction can be repeated, depending upon the conditions, until all of the alkoxide groups have been replaced.
The second reaction is the condensation of the hydroxy-containing species to form metal-oxygen-metal bonds (eq. 2). EQU 2 HO-SI-(OCH.sub.2 CH.sub.3).sub.3 .fwdarw.(CH.sub.3 CH.sub.2 O).sub.3 -Si-O-Si-(OCH.sub.2 CH.sub.3).sub.3 +H.sub.2 O
This reaction can proceed until all of the hydroxide groups have been used up, resulting in a network of ceramic-type bonds (eq. 3). EQU n (CH.sub.3 CH.sub.2 O).sub.3 -Si-O-Si-(OCH.sub.2 CH.sub.3).sub.3 .fwdarw.2n[SiO.sub.2 ]
The chemistry of sol-gel processing is well documented in the prior art. See, e.g., (1) Brinker et al, "Sol-gel Transition in Simple Silicates", J. Non-Cryst. Solids, 48 (1982),47-64; (2) Brinker et al, "Sol-gel Transition in Simple Silicates II", J. Non-Cryst. Solids, 63 (1984) 45-59; (3) Schaefer et al, "Characterization of Polymers and Gels by Intermediate Angle X-ray Scattering", presented at the International Union of Pure and Applied Chemists MACRO'82, Amherst, Mass., July 12, 1982; (4) Pettit et al, Sol-Gel Protective Coatings for Black Chrome Solar Selective Films, SPIE Vol. 324, Optical Coatings for Energy Efficiency and Solar Applications (pub. by the Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash.) (1982) 176-183; (5) Brinker et al, "Relationships Between the Sol to Gel and Gel to Glass Conversions", Proceedings of the International Conference on Ultrastructure Processing of Ceramics, Glasses, and Composites, (John Wiley and Sons, N.Y.) (1984); (6) Brinker et al, "Conversion of Monolithic Gels to Glasses in a Multicomponent Silicate Glass System", J. Materials Sci., 16 (1981) 1980-1988; (7) Brinker et al, "A Comparison Between the Densification Kinetics of Colloidal and Polymeric Silica Gels", Mat. Res. Soc. Symp. Proc. Vol. 32 (1984), 25-32; all of which disclosures are incorporated by reference herein.
One of the drawbacks of these prior art sol-gel processes is that formation of thin coatings of multiple layers on a substrate requires multiple sequential immersions of the substrate into different coating solutions.
It is an object of the present invention to provide a substrate with a thin film coating of multiple layers by a single immersion of the substrate into a coating solution, thereby avoiding multiple sequential immersions of the substrate into different coating solutions to form a multiple layer coating.
It is another object of the invention to provide a substrate with a thin film coating of multiple layers which coating inhibits corrosion of the substrate.
It is another object of the invention to provide a substrate with a thin film coating of multiple layers which coating improves adhesion in substrate adhesive bonding applications.
It is another object of the invention to provide a substrate with a thin film coating of multiple layers which coating is complete and uniform even at thicknesses ranging from 5 to 500 A.
It is another object of the invention to provide a substrate with a thin film coating of multiple layers which coating has physical and chemical properties which vary as a function of the depth into the coating.